Nonaqueous liquid electrolyte and nonaqueous liquid electrolyte secondary battery

ABSTRACT

Disclosed is a nonaqueous liquid electrolyte comprising a nonaqueous solvent, an electrolyte dissolved in the nonaqueous solvent and a macromolecular material added to the nonaqueous solvent. The nonaqueous liquid electrolyte is a fluid having a viscosity at 20° C. of 7 cP to 30,000 cP. The nonaqueous liquid electrolyte suppresses leakage, ensures high discharge characteristics, reduces the unevenness of liquid electrolyte, and lessens the change of electrodes and the change in battery resistivity.

BACKGROUND OF THE INVENTION

The present invention relates to a nonaqueous liquid electrolyte and anonaqueous liquid electrolyte secondary battery

Currently, a lithium ion secondary battery has been commercialized as anonaqueous liquid electrolyte secondary battery intended for a portabledevice such as a portable cellular phone. This particular battery has apositive electrode, a negative electrode and a separator which containsa liquid electrolyte, wherein utilized are lithium cobalt oxide (LiCoO₂)for the positive electrode, a graphitic or carbonaceous material for thenegative electrode, an organic solvent in which a lithium salt has beendissolved for the nonaqueous liquid electrolyte and a porous membranefor the separator.

The nonaqueous liquid electrolyte is a nonaqueous solvent in which anelectrolyte has been dissolved, for which used usually is a lowviscosity and low boiling point material such as a liquid mixturecomprising propylene carbonate, ethylene carbonate or γ-butyrolactone,etc.

In the meantime, the nonaqueous liquid electrolyte secondary battery isadapted to be mounted on a portable device as being housed in ahermetically sealed container or the like. In so doing, a problem arisesin which the nonaqueous liquid electrolyte may leak out of thehermetically sealed container. The battery also has a disadvantage thatits electrodes may deform after charge/discharge cycles to largely swellits outer packaging.

To overcome such problems, a gel-like electrolyte has been proposed toameliorate such leakage, which contains a solid electrolyte as an ionconducting material instead of using a nonaqueous liquid electrolyte.See, for example, Japanese Patent Unexamined Publication 2000-315523.

Since such a gel-like electrolyte contains a large amount ofmacromolecular material, however, the resin that is the base thereforwill severely prevent the movement of the electrolyte, therebyremarkably reducing the ion conductivity as compared with the case of anonaqueous liquid electrolyte alone. There has also been a disadvantagethat contact with the electrodes will lessen as compared with the caseof a liquid, which increases the resistance at the interface of theelectrodes, thereby deteriorating the discharge characteristics of thenonaqueous liquid electrolyte secondary battery.

Furthermore, since a gel-like electrolyte is produced by encapsulatingan electrolyte and a monomer together with electrodes in an outerpackaging for the battery and then filling it with a gelling agent, theelectrolyte will become gel-like while the gelling agent infiltratesfrom around the outer packaging toward the center. As such, there is adisadvantage that a homogeneous gel electrolyte may not easily beobtained between the electrodes and the electrodes per se willconsiderably deform during the charge/discharge cycles to swell theouter packaging.

As described above, a nonaqueous liquid electrolyte secondary batteryutilizing a gel-like electrolyte containing a nonaqueous liquidelectrolyte in order to prevent leakage or the like has been unable toprovide sufficient discharge characteristics. In addition, aconsiderable amount of deformation will result in association with thecharging and discharging

BRIEF SUMMARY OF THE INVENTION

Accordingly, in the light of the aforementioned disadvantages, an objectof the present invention is to provide a nonaqueous liquid electrolyteand a nonaqueous liquid electrolyte secondary battery having highdischarge characteristics and suppressed leakage wherein deformation ofits electrodes may hardly occur.

Another object of the present invention is to provide a nonaqueousliquid electrolyte secondary battery wherein a liquid electrolyte can bedistributed evenly over the surface of its electrodes and ionconductivity will not be reduced.

According to the present invention, there is provided a nonaqueousliquid electrolyte comprising a nonaqueous solvent, an electrolytedissolved in the nonaqueous solvent and a macromolecular material addedto the nonaqueous solvent, wherein the nonaqueous liquid electrolyte isa fluid having a viscosity at 20° C. of 7 cP to 30,000 cP. To allow fordeformation of electrodes, it is desirably a fluid having a viscosity of50 cP or higher.

There is further provided a nonaqueous liquid electrolyte comprising anelectrolyte and a macromolecular material both added to a nonaqueoussolvent, wherein the nonaqueous liquid electrolyte at 20° C. is a fluidwhich exhibits non-Newtonian properties.

There is also provided a nonaqueous liquid electrolyte comprising anelectrolyte and a macromolecular material both added to a nonaqueoussolvent, wherein the ratio of ion conductivity σ (10⁻³ S/cm) toviscosity η (cP), p (σ/η), in the nonaqueous liquid electrolyte at 20°C. is <0.1.

Furthermore, according to the present invention, there is provided anonaqueous liquid electrolyte secondary battery comprising a positiveelectrode containing an active material, a negative electrode containinga material which absorbs and desorbs lithium ions and a liquidelectrolyte sandwiched between the positive and negative electrodes,wherein the liquid electrolyte comprises a nonaqueous solvent containingγ-butyrolactone, an electrolyte dissolved in the nonaqueous solvent anda macromolecular material comprising the structure represented by theformula:

CH₂—CH₂—O

_(n)

wherein n≧1, which is added to the nonaqueous solvent, the content ofthe macromolecular material being 0.01% or more but less than 10% byweight.

There is further provided a nonaqueous liquid electrolyte secondarybattery comprising a positive electrode containing an active material, anegative electrode containing a material which absorbs and desorbslithium ions and a liquid electrolyte sandwiched between the positiveand negative electrodes, wherein a macromolecular material which, addedin an amount of 0.01% or more but less than 10% by weight, brings theviscosity of the nonaqueous liquid electrolyte at 20° C. within therange of 7 cP to 30,000 cP is added to the nonaqueous liquid solvent.

There is also provided a nonaqueous liquid electrolyte secondary batterycomprising a positive electrode containing an active material, anegative electrode containing a material which absorbs and desorbslithium ions and a liquid electrolyte sandwiched between the positiveand negative electrodes, wherein the nonaqueous liquid electrolyteconsists of a nonaqueous solvent, an electrolyte dissolved in thenonaqueous solvent and a macromolecular material added to the nonaqueoussolvent, and the nonaqueous liquid electrolyte at 20° C. is a fluidwhich exhibits non-Newtonian properties.

There is finally provided a nonaqueous liquid electrolyte secondarybattery comprising a positive electrode containing an active material, anegative electrode containing a material which absorbs and desorbslithium ions and a liquid electrolyte sandwiched between the positiveand negative electrodes, wherein the ratio of ion conductivity a (10⁻³S/cm) to viscosity η (cP), p (σ/η), in the nonaqueous liquid electrolyteat 20° C. is <0.1.

The inventors thought that the leakage of medium from a battery could beprevented by using a high viscosity, nonaqueous liquid electrolyte as anion conducting material instead of using a gel-like electrolyte and, asa result of intensive research, have found out that the viscosity of anonaqueous liquid electrolyte can be extremely increased by adding a few% by weight of polyethylene oxide to the nonaqueous liquid electrolytewhich uses y-butyrolactone as a nonaqueous solvent. In other words, theyhave found out that by adding to the nonaqueous liquid electrolyte asmall amount of macromolecular material as appropriately selecteddepending on the nonaqueous solvent, it will be possible to increase theviscosity of the nonaqueous liquid electrolyte and, consequently, toprevent the leakage of the nonaqueous liquid electrolyte from thebattery while inhibiting the movement of the electrolyte in thenonaqueous liquid electrolyte and improving the characteristics of thenonaqueous liquid electrolyte secondary battery.

In the light of a problem that since the liquid electrolyteconventionally used has been a fluid which exhibits non-Newtonianproperties, an uneven distribution of the electrolyte occurs within abattery during cycles, involving the distortion of its electrodes, theinventors have further found out that by using a high viscosity,nonaqueous liquid electrolyte, specifically, a fluid which exhibitsnon-Newtonian properties at 20° C., the electrolyte can be distributedevenly within a nonaqueous liquid electrode secondary battery duringcycles and the distortion of its electrodes can be suppressed.

As such a non-Newtonian fluid, the one corresponding to any of themodels below is particularly desirable:

1) Bingham Model

This is a fluid in which deformation of the fluid occurs when the shearstress τ across the fluid is larger than the yield stress τy, and arelationship is established between τ and the shear rate of the fluid,γ, as:τ=τy+η ₀ γ=τy+η ₀ du/dy, when τ>τyγ=du/dy=0, when τ>τy.

2) Power Law Model

This is a fluid in which a relationship is established as:τ=Kγ ^(n) =K(du/dy)^(n).

Designate a fluid with n>1 a dilatant fluid and a fluid with n<1 apseudoplastic fluid. When n<1, the greater γ is, the gentler the slopebetween the shear stress τ and the shear rate γ is. A power law fluidwith n<1 is designated a pseudoplastic fluid since the flow curve isapproximated in the region where γ is great and, at γ=0, apparentplasticity which is finite shear stress appears.

3) Herschel-Bulkey Model

This is a combination of Bingham and power law models, as representedby:τ=τy+Kγ ^(n) =τy+K(du/dy)^(n).

In addition, for a non-Newtonian fluid, the apparent viscosity isassigned to a case where it decreases with the increase in the shearrate (shear thinning) and a case where it increases with the increase inthe shear rate (shear thickening). In the nonaqueous liquid electrolyteaccording to the present invention, it should preferably be shearthinning wherein it decreases with the increase in the shear rate. Itshould also desirably be a Bingham plastic fluid or a pseudoplasticfluid (a power law fluid with n<1).

In addition, since conventionally used liquid electrolytes are lowviscosity fluids, which tend to be offset within batteries duringcycles, there has been a disadvantage of uneven reactions on electrodes.That disadvantage is attributable to the fact that in a lithium ionbattery using a conventional organic liquid electrolyte, increasing theion conductivity σ (10⁻³ S/cm) of the liquid electrolyte whiledecreasing the viscosity ρ (cP), that is, increasing the ratio betweenthem, p (σ/η), has been supposed to be a factor for improving thebattery characteristics.

Based on a consideration that uniform reactions are made possible onelectrodes by using a high viscosity, nonaqueous liquid electrolytehaving a high ion conductivity, the inventors have found out that bydeveloping a nonaqueous liquid electrolyte secondary battery having anonaqueous liquid electrolyte with a ratio of the ion conductivity σ(10⁻³ S/cm) to the viscosity η (cP), p (σ/η), of <0.1, it is possible tocause uniform reactions on the electrodes and to remarkably suppress thereaction resistance. The range of p should desirably be 0.0001 or morebut less than 0.1, preferably 0.0005 to 0.08 and more preferably 0.001to 0.05.

A nonaqueous liquid electrolyte secondary battery according to thepresent invention will now be described in more detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate a presently preferred embodimentof the invention, and together with the general description given aboveand the detailed description of the preferred embodiment given below,serve to explain the principles of the invention.

FIG. 1 is a sectional view of a thin, lithium ion battery illustratingan embodiment of a nonaqueous liquid electrolyte secondary batteryaccording the present invention; and

FIG. 2 is an enlarged section of portion A taken from FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, a nonaqueous liquid electrolyte secondary batteryaccording to the present invention comprises an outer packaging material1 made of a film for example and an electrode assembly 2 (a group ofelectrodes) enclosed by the outer packaging material. The electrodeassembly 2 has a construction in which a laminate consisting of positiveelectrodes, separators and negative electrodes is spirally wound into aflattened shape.

As shown in FIG. 2, the electrode assembly 2 wound into the flattenedshape consists of a separator 3, a positive electrode 12, anotherseparator 3, a negative electrode 13, another separator 3, anotherpositive electrode 12, another separator 3 and another negativeelectrode, which are stacked in this order from below as seen in thedrawing.

Each of the negative electrodes 13 has a three layer construction inwhich a negative electrode layer 6, a negative electrode collector 7 andanother negative electrode layer 6 are stacked in this order while eachof the positive electrodes 12 has a three layer construction in which apositive electrode layer 4, a positive electrode collector 5 and anotherpositive electrode layer 4 are stacked in this order. The outermostnegative electrode 13, however, has a two layer construction of anegative electrode layer 6 and a negative electrode collector 7 asreferred to from below in the drawing and, at the outermost side, thenegative electrode collector 7 is attached to the outer packagingmaterial 1 via an adhesive layer 8. A nonaqueous liquid electrolyte isinjected into the outer packaging material 1 and retained in theseparators.

A strip-like positive electrode lead 10 has one end which is connectedto the positive electrode collector 5 of the electrode assembly 2 andthe other end which extends out of the outer packaging material 1. Onthe other hand, a strip-like negative electrode lead 11 has one endwhich is connected to the negative electrode collector 7 of theelectrode assembly 2 and the other end which extends out of the outerpackaging material 1. The thin, nonaqueous liquid electrolyte secondarybattery can be produced by accommodating the electrode assembly as suchin the outer packaging material followed by injecting a nonaqueousliquid electrolyte before sealing the opening, etc.

Each component of such a nonaqueous liquid electrolyte secondary batterywill then be described in more detail below.

(1) The Positive Electrode

The positive electrode has a collector for positive electrode and apositive electrode layer formed on one or both sides of the collector.As the collector, a conductive substrate of a porous structure or anonporous conductive substrate may be used. Such conductive substratesmay be formed of aluminum, stainless steel or nickel, for example.

The positive electrode layers contain a positive electrode activematerial, and are usually formed of a composite material furthercontaining a conductor agent and a binder resin. As the positiveelectrode active material, various kinds of oxides, such as manganesedioxide, lithium/manganese composite oxide, lithium-containing nickeloxide, lithium-containing cobalt oxide, lithium-containing nickel cobaltoxide, lithium-containing iron oxide and lithium-containing vanadiumoxide as well as chalcogen compounds such as titanium disulfide andmolybdenum disulfide may be mentioned. Among those mentioned,lithium-containing cobalt oxide such as LiCoO₂, lithium-containingnickel cobalt oxide such as LiNi_(0.8)Co_(0.2)O₂ and lithium/manganesecomposite oxide such as LiMn₂O₄ and LiMnO₂ are preferable for theirability to obtain a high voltage.

As the conductor agent, acetylene black, carbon black, graphite, etc.may be mentioned, for example. The binder functions to retain the activematerial on the collector and to connect the active material with eachother. As the binder, polytetrafluoroethylene (PTFE), polyvinylidenefluoride (PVdF), ethylene-propylene-diene copolymer (EPDM),styrene-butadiene rubber (SBR), etc. can be employed, for example. Themixing ratios of the positive electrode active material, conductor agentand binder should preferably be 80% to 95% by weight, 3% to 20% byweight and 2% to 7% by weight, respectively.

(2) The Negative Electrode

The negative electrode has a collector for negative electrode and anegative electrode layer formed on one or both sides of the collector.As the collector for negative electrode, a conductive substrate of aporous structure or a nonporous conductive substrate may be used. Suchconductive substrates may be formed of copper, stainless steel ornickel, for example.

The negative electrode layer contains a material which is capable ofabsorbing or desorbing lithium ions, and is usually formed of acomposite material further containing a conductor agent and a binderresin. As the material which is capable of absorbing or desorbinglithium ions, a graphitic or carbonaceous material, such as graphite,coke, a carbon fiber, a globular fiber or the like as well as agraphitic or carbonaceous material which is obtainable by heat treatinga thermoplastic resin, isotropic pitch, mesophase pitch, mesophasepitch-based carbon fiber, mesophase globule or the like (in particular,a mesophase pitch-based carbon fiber is preferable for its ability toimprove capacity and charge/discharge cycle characteristics) at 500 to3,000° C. etc. may be mentioned. Among those mentioned, a graphiticmaterial which is obtained by increasing the temperature for theaforementioned heat treatment to 2,000° C. or higher and has graphitecrystals whose interplanar spacing d₀₀₂ of (002) planes is 0.340 nm orless should preferably be used. A nonaqueous liquid electrode secondarybattery having a negative electrode which contains such a graphiticmaterial as a carbonaceous material can considerably increase thebattery capacity and the bulk current discharge characteristics. Morepreferably, the spacing doo₂ is 0.336 nm or less.

As the material which absorbs and desorbs lithium ions, a metal such asaluminum, magnesium, tin, silicon etc., a metal compound selected frommetal oxides, metal sulfides and metal nitrides or a material containinga lithium alloy may also be suitable. As the metal oxide, tin oxide,silicon oxide, lithium titanium oxide, niobium oxide, tungsten oxide,etc. may be mentioned. As the metal sulfide, tin sulfide, titaniumsulfide, etc. may be mentioned, for example. As the metal nitride,lithium cobalt nitride, lithium iron nitride, lithium manganese nitride,etc. may be mentioned, for example.

As the lithium alloy, lithium/aluminum alloy, lithium/tin alloy,lithium/lead alloy, lithium/silicon alloy, etc. may be mentioned, forexample.

As the binder, polytetrafluoroethylene (PTFE), polyvinylidene fluoride(PVdF), ethylene-propylene-diene copolymer (EPDM), styrene-butadienerubber (SBR), carboxymethyl cellulose (CMC), etc may be employed, forexample.

The mixing ratio of the carbonaceous material and the binder shouldpreferably be 90% to 98% by weight for the carbonaceous material and 2%to 20% by weight for the binder.

(3) The Separator

The separator is for preventing short-circuiting between the positiveand negative electrodes, and is formed of an insulating material. Anonaqueous liquid electrolyte is retained in the separator and a poroussheet provided with pores is used so that lithium ions can move betweenthe electrodes.

As the porous sheet, a porous film or a non-woven fabric may be used,for example. The porous sheet should preferably be made of at least onematerial selected from polyolefins and celluloses, for example. As thepolyolefins, polyethylene and polypropylene may be mentioned, forexample. Among those mentioned, a porous film made of polyethylene,polypropylene or both is preferable for its ability to improve thesafety of secondary batteries.

The thickness of the porous sheet should preferably be 30 μm or less. Ifthe thickness exceeds 30 μm, then it may disadvantageously increase thespacing between the positive and negative electrodes, thereby enlargingthe internal resistance. The lower limit of the thickness shouldpreferably be 5 μm. If the thickness is less than 5 W, then the strengthof the separator may remarkably decrease, thereby increasing thepossibility of causing internal short-circuiting. The upper limit of thethickness should more preferably be 25 μm while the lower limit of thethickness should more preferably be 10 μm.

As the porous sheet, a porous material having a porosity in the range of30% to 60% should desirably be used. When the porosity is less than 30%,the amount of liquid electrolyte to be retained within the porous sheetwill lessen, thereby decreasing the ion conductivity. If the porosityexceeds 60%, the mechanical strength will then be insufficient.

When a gel-like electrolyte is used as an ion conducting material, thegel-like electrolyte must directly be sandwiched between electrodessince it is difficult to form a gel-like electrolyte within pores of aporous material. When a nonaqueous liquid electrolyte is used as an ionconducting material, however, the electrolyte can be retained within thepores of the mechanically strong porous material. As such,short-circuiting between electrodes, etc. can be prevented

(4) The Nonaqueous Liquid Electrolyte The nonaqueous liquid electrolytehas a nonaqueous solvent and an electrolyte and macromolecular materialboth dissolved in the nonaqueous liquid electrolyte.

4-1) The Electrolyte

As the electrolyte to be incorporated into the nonaqueous liquidelectrolyte, a lithium salt (electrolyte) such as lithium perchlorate(LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride(LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithiumtrifluorometasulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimidelithium [LiN(CF₃SO₂)₂], etc. may be mentioned, for example. Among thosementioned, it is preferable to use either LiPF₆ or LiBF₄. The content ofsuch electrolytes dissolved in the nonaqueous solvent should desirablybe 0.5 to 2.0 mol/l.

4-2) The Nonaqueous Solvent

The nonaqueous solvent is selected depending on the macromolecularmaterial used and can be improved in its viscosity by adding a smallamount of the selected macromolecular material. For instance, thenonaqueous solvent in case polyethylene oxide is used as themacromolecular material will be described. For the nonaqueous solvent,an organic solvent containing γ-butyrolactone (BL) is to be used.Although BL alone can be used as the nonaqueous solvent, it ispreferable to use a mixed nonaqueous solvent consisting mainly of BL.Specifically, it is preferable to use a mixed nonaqueous solventcontaining 50% to 95% by volume of BL.

If the ratio of BL in the nonaqueous solvent is less than 50% by volume,a gas may then tend to be generated at a high temperature. If the ratioof BL exceeds 95% by volume, a reaction will then occur between thenegative electrode and BL, thereby decreasing the charge/discharge cyclecharacteristics. When a carbonaceous material is used for the negativeelectrode, for example, the carbonaceous material will react with BL tocause a reductive decomposition of the nonaqueous liquid electrolyte,forming on the surface of the negative electrode a coating whichinhibits the charge/discharge reactions Consequently, electric currenttends to concentrate on the negative electrode so that lithium metal maydeposit on the surface of the negative electrode or the impedance overthe interface of the negative electrode may increase, thereby reducingthe charge/discharge efficiency of the negative electrode anddeteriorating the charge/discharge cycle characteristics.

A more preferable ratio of BL in the nonaqueous solvent is 60% to 95% byvolume. Bringing it within this range can further enhance the effect ofinhibiting gas generation during storage at a high temperature and canfurther improve the discharge capacity under a low temperatureenvironment at around −20° C. An even more preferable ratio is 65% to90% by volume. As the nonaqueous solvent to be mixed with BL, a cycliccarbonate is desirable for its ability to improve the charge/dischargeefficiency of the negative electrode. It may further include a lowviscosity solvent such as a chain carbonate, chain ether, cyclic ether,etc. in an amount of 20% by volume or less.

As the cyclic carbonate, propylene carbonate (PC), ethylene carbonate(EC), vinylene carbonate (VC), trifluoropropylene carbonate (TFPC), etc.are desirable. In particular, when EC is used as a solvent to be mixedwith BL, the charge/discharge characteristics and the bulk currentdischarge characteristics can be greatly improved. As other solvents tobe mixed with BL, mixed solvents of a third solvent consisting of atleast one selected from the group consisting of PC, VC, TFPC, diethylcarbonate (DEC), methylethyl carbonate (MEC) and aromatic compounds andEC would be desirable for their ability to improve the charge/dischargecycle characteristics.

More specific examples of the composition of the mixed nonaqueoussolvent are BL and EC; BL and PC; BL, EC and DEC; BL, EC and MEC; BL,EC, MEC and VC; BL, EC and VC; BL, PC and VC; and BL, EC, PC and VC.

When the mixed nonaqueous solvent of EL and EC is used, the volume ratioof EC should preferably be 5% to 40% by volume. If the ratio of EC isless than 5% by volume, it may be difficult to cover the surface of thenegative electrode closely with a protective film, so that a reactionbetween the negative electrode and BL may occur, thus making itdifficult to sufficiently improve the charge/discharge cyclecharacteristics. If the ratio of EC exceeds 40% by volume, on the otherhand, the viscosity of the nonaqueous liquid electrolyte may increase todecrease the ion conductivity, so that it may become difficult tosufficiently improve the charge/discharge cycle characteristics, bulkcurrent discharge characteristics and low temperature dischargecharacteristics. A more preferable range of the ratio of EC is 10% to35% by volume.

When at least one selected from DEC, MEC, PC and VC is used as acomponent of the mixed nonaqueous solvent, it will have the effect offorming a close protective film over the surface of the negativeelectrode and thereby decreasing the interface impedance of the negativeelectrode. The amount of this solvent is not particularly limited and isto be defined at such an amount that this effect may occur. If the ratioof these components in the mixed nonaqueous solvent exceeds 10% byvolume, however, it may be difficult to sufficiently inhibit theoxidative decomposition of the nonaqueous liquid electrolyte under ahigh temperature environment or the viscosity of the nonaqueous liquidelectrolyte may increase to decrease the ion conductivity. With this inmind, the ratio by volume of these components in the mixed nonaqueoussolvent should desirably be 10% by volume or less. A more preferableratio by volume is 2% by volume or less. The lower limit of the ratio byvolume should preferably be 0.001% by volume and more preferably 0.05%by volume.

Among the compositions of the aforementioned mixed nonaqueous solvents,in particular, a mixed nonaqueous solvent which comprises more than 50%but up to 95% by volume of BL and additive amounts of EC and VC ispreferable. A nonaqueous liquid electrolyte secondary battery having anonaqueous liquid electrolyte comprising this nonaqueous solvent and anegative electrode comprising a carbonaceous material which absorbs anddesorbs lithium ions can largely decrease the impedance over theinterface of the negative electrode and suppress the deposition oflithium ions on the negative electrode, thereby improving thecharge/discharge efficiency of the negative electrode. Consequently, gasgeneration at high temperature storage can be inhibited to suppress thedeformation of the outer packaging material while realizing excellentbulk current discharge characteristics and a long life. Such improvementof the negative electrode characteristics may assumedly be due theeffects as described below. In the aforementioned secondary battery, thesurface of the negative electrode is covered with a protective film byEC and, in addition, a thin, close coating is formed by VC. It isconsequently assumed that any reactions between BL and the negativeelectrode can further be suppressed so that the reduction of impedanceand the prevention of the deposition of lithium metal can be attained.

In addition, as the nonaqueous solvent, a mixed nonaqueous solvent whichcomprises more than 50% but up to 95% by volume of BL and additiveamounts of EC and an aromatic compound may be used in place of onehaving the aforementioned composition. As the aforementioned aromaticcompounds, at least one selected from benzene, toluene, the xylenes,biphenyl and terphenyl may be mentioned, for example. EC can attach tothe surface of the negative electrode (for example, one containing acarbonaceous material which absorbs and desorbs lithium ions) and form aprotective film to inhibit a reaction between the negative electrode andBL. The ratio by volume of EC should then preferably be 5% to 40% byvolume, for the same reasons as described above. A more preferable rangefor the ratio of EC is 10% to 35% by volume. In the meantime, thebenzene rings of the aforementioned aromatic compounds can easily adsorbto the surface of the negative electrode (for example, one containing acarbonaceous material which absorbs and desorbs lithium ions) totherefore inhibit a reaction between the negative electrode and BL.Thus, the nonaqueous liquid electrolyte containing the nonaqueoussolvent comprising more than 50% but up to 95% by volume of BL as wellas EC and an aromatic compound can sufficiently inhibit reactionsbetween the negative electrode and BL, thereby improving thecharge/discharge cycle characteristics of a secondary battery.

When DEC, MEC, PC, TFPC or VC is included as a component of the mixednonaqueous solvent, the charge/discharge cycle characteristics canfurther be improved, since it may further inhibit reactions between thenegative electrode and BL. VC is preferable among them. The amount ofthe third solvent consisting of at least one selected from aromaticcompounds, DEC, MEC, PC, TFPC and VC is not particularly limited and isto be defined at such an amount that this effect may occur. If the ratioof the third solvent in the nonaqueous solvent exceeds 10% by volume,however, it may be difficult to sufficiently inhibit the oxidativedecomposition of the nonaqueous liquid electrolyte under a hightemperature environment or the viscosity of the nonaqueous liquidelectrolyte may increase to decrease the ion conductivity. With this inmind, the ratio by volume of the third solvent in the nonaqueous solventshould desirably be 10% by volume or less. A more preferable ratio byvolume is 2% by volume or less. The lower limit of the ratio by volumeshould preferably be 0.001% by volume and more preferably 0.05% byvolume.

4-3) The Macromolecular Material

The macromolecular material dissolves in the nonaqueous solvent tomaintain a high lithium ion conductivity and improve the viscosity ofthe nonaqueous liquid electrolyte obtained. The polyacrylonitrile (PAN)series, polyacrylate (PMMA) series, polyvinylidene fluoride (PVdF)series, polyvinyl chloride (PVC) series, polyethylene oxide series, etc.may be used. Depending on the type of the nonaqueous solvent in whichthey are dissolved, however, the degree of increase in viscosityrelative to the amount dissolved will differ. When an organic solventcontaining BL as a nonaqueous solvent is used, it is preferable to use amacromolecule of the PEO series comprising the structure represented bythe formula:

CH₂—CH₂—O

_(n)

wherein n≧1.

Since the PEO series macromolecule, dissolved in BL in a slight amount,can remarkably increase the viscosity of the nonaqueous liquidelectrolyte obtained, not only the movement of the electrolyte in theliquid electrolyte by a macromolecular material does not have to beimpaired but, since it exists evenly throughout the nonaqueous liquidelectrolyte, the ion conductivity may also be improved further.

The PEO series macromolecule should preferably be present in an amountof 0.01% or more but less than 10% by weight, on the basis of theaforementioned nonaqueous liquid electrolyte containing BL. If theamount of the PEO series macromolecule is less than 0.01% by weight, itwill then be likely that the nonaqueous liquid electrolyte leaks outthrough the outer packaging material. If the amount of the PEOmacromolecule is 10% by weight or more, on the other hand, the lithiumion conductivity of the secondary battery will then remarkably decrease,making it difficult to improve the discharge capacity, bulk currentdischarge characteristics and charge/discharge cycle characteristics.The range of n should preferably be 2×10¹≦n≦2×10⁶ and more preferably1×10²≦n≦1×10⁶.

The amount of the nonaqueous liquid electrolyte should preferably be 0.2to 0.6 g per 100 mA of battery unit capacity, for the reasons such asbelow. If the amount of the nonaqueous liquid electrolyte is less than0.2 g/100 mAh, then it may not be possible to maintain a sufficient ionconductivity across the positive and negative electrodes. If the amountof the nonaqueous liquid electrolyte exceeds 0.6 g/100 mAh, on the otherhand, the amount of the electrolyte will be so large that it may becomedifficult to seal with an outer packaging material made of a film. Amore preferable range of the amount of the nonaqueous liquid electrolyteis 0.4 to 0.55 g/100 mAh.

The average molecular weight of the macromolecular material shouldpreferably be in the range of 1×10³ to 1×10⁸. Outside this range, it maybe impossible to enhance the viscosity of the nonaqueous liquidelectrolyte with the addition in a small amount of the macromolecularmaterial.

Thus, adding a macromolecular material will bring the viscosity of theliquid electrolyte obtained within the range of 7 cP to 30,000 cP. Ifthe viscosity of the liquid electrolyte is lower than 7 cP, the liquidelectrolyte may then leak out through the outer packaging material andif it is higher than 30,000 cP, it will then be difficult to impregnatethe separator with the liquid electrolyte.

By increasing the viscosity of the liquid electrolyte, deformation ofthe electrode assembly which occurs when charging and discharging arerepeated may be prevented and, in turn, by preventing such deformation,swelling of the outer packaging may be reduced and the capacityretention ratio may be improved.

(5) The Outer Packaging Material

As the outer packaging material, a sheet comprising a metal layer andresin layers coated on both sides of the metal layer, 0.5 mm or less inthickness including the resin layers, is used. This outer packagingmaterial is light in weight so that the energy density may be increasedper weight of battery; however, it is susceptible, because of itsflexibility, to deformation due to gases generated from the electrodeassembly and the nonaqueous liquid electrolyte.

The resin layers act as protective layers for the metal layer and may beformed of, for example, polyethylene, polypropylene, etc. The metallayer is responsible for shutting off moisture. As the metal layer,aluminum, stainless steel, iron, copper, nickel, etc. may be mentioned,for example. Among those mentioned, aluminum which is light in weightand effective in shutting off moisture is preferable. The metal layermay be formed of one metal or may be formed of two or more metalsintegrated into a layer. Of the resin layers formed on both sides of themetal layer, one provided on the outside of the battery is responsiblefor preventing damages to the metal layer. This outer resin layer may beformed of one resin or may be formed of two or more resins. The otherresin layer provided on the inside of the battery is responsible forpreventing the metal layer from being corroded by the nonaqueous liquidelectrolyte. This inner resin layer may be formed of one resin or may beformed of two or more resins.

If the thickness of the outer packaging material exceeds 0.5 mm, thecapacity per weight of battery will then decrease. The thickness of theouter packaging material should preferably be 0.3 mm or less, morepreferably 0.25 mm or less and most preferably 0.15 mm or less. If thethickness is less than 0.05 mm, deformation or breakage will then tendto occur. Thus, the lower limit of the thickness should preferably be0.05 mm. The lower limit should more preferably be 0.08 mm and mostpreferably 0.1 mm.

The thickness of the outer packaging material is to be measured by themethod to be described below. Specifically, three points mutuallyseparated by 1 cm or more over the area of the outer packaging materialexcept where the seal of the material is made are selected at random, athickness is measured at each point and the average is calculated andreferred to as the thickness of the outer packaging material. If anyforeign matters (resin, for example) are attached on the surface of theouter packaging material, such matters must then be removed beforetaking measurement of the thickness.

After assembling such a lithium ion secondary battery, an initial chargeis carried out at a charge rate of 0.05 C to 0.5 C under a temperaturecondition of 30° C. to 80° C. Under these conditions, the charge may becarried out for only one cycle or may be carried out for 2 or morecycles. The battery may be stored for 1 to 100 hours under a temperaturecondition of 30° C. to 80° C. before charging. 1 C charge rate hereinrefers to the current value required to charge a nominal capacity (Ah)in one hour.

The temperature during the initial charge is to be set within theaforementioned range for the following reasons. If the initial chargetemperature is below 30° C., it will then be difficult to impregnate thepositive and negative electrodes and the separator evenly with thenonaqueous liquid electrode since the viscosity of the nonaqueous liquidelectrolyte remains high, so that the internal impedance will increaseand the utilization of the active material will decrease. If the initialcharge temperature exceeds 80° C., the binder contained in the positiveand negative electrodes will then deteriorate.

By bringing the charge rate for the initial charge within the range of0.05 C to 0.5 C, it is possible to moderately delay the expansion of thepositive and negative electrodes by charging so that the positive andnegative electrodes may be infiltrated evenly with the nonaqueous liquidelectrolyte.

EXAMPLES Example 1

(Fabrication of Positive Electrode)

First, 92% by weight of lithium cobalt oxide (Li_(x)CoO₂, wherein 0≦X≦1)powder, 3% by weight of acetylene black, 3% by weight of graphite and 2%by weight of ethylene-propylene-diene monomer powder were mixed togetherwith toluene and then the mixture was applied to both sides of a currentcollector made of a porous aluminum foil (15 μm in thickness) havingpores 0.5 mm in diameter at a concentration of 10 pores per 10 cm²,followed by press-working, to produce a positive electrode having anelectrode density of 3.2 g/cm³ in which positive electrode layers werecarried on both sides of the collector.

(Fabrication of Negative Electrode)

95% by weight of mesophase pitch-based carbon fiber (fiber diameter 8μm, average fiber length 18 μm and average interplanar spacing (d₀₀₂)0.3360 nm) powder as a carbonaceous material was mixed with 5% by weightof polyvinylidene fluoride (PVdF) as a binder and the mixture wasapplied to a current collector made of a copper foil (15 μm inthickness), dried and press-worked to produce a negative electrodehaving an electrode density of 1.7 g/cm³ in which negative electrodelayers were carried on both sides of the collector.

(Separator)

A separator, 20 μm in thickness, 20% in heat shrinkage at 120° C. for1.5 hours and 55% in porosity, consisting of a porous film made ofpolyethylene was provided.

(Preparation of Nonaqueous Liquid Electrolyte)

Into a mixed solvent of ethylene carbonate (EC) and γ-butyrolactone(volume ratio 25:75) 1.5 mol/l of lithium borofluoride (LiBF₄) wasdissolved to prepare a nonaqueous liquid electrolyte. Then, 0.7% byweight, on the basis of the nonaqueous liquid electrolyte, ofpolyethylene oxide having a molecular weight of 5,000,000 was added withstirring to the liquid electrolyte to prepare a nonaqueous liquidelectrolyte.

(Fabrication of Electrode Assembly)

A strip-like positive electrode lead was welded to the collector of thepositive electrode and a strip-like negative electrode lead was weldedto the collector of the negative electrode. Then, the positive andnegative electrodes were spirally wound with the separator between them,followed by shaping into a flattened form to fabricate an electrodeassembly.

A laminated film, 90 μm in thickness, made of an aluminum foil coveredon both sides with polypropylene was shaped into a sack, into which theelectrode assembly was accommodated in such a manner that the laminatedsurface shown in FIG. 2 could be seen through the opening of the sack.

The nonaqueous liquid electrolyte was injected into the electrodeassembly in the laminated film so that the amount reached 4.5 g per Ahof battery capacity to assemble a thin, nonaqueous liquid electrolytesecondary battery, 3 mm in thickness, 40 mm in width and 70 mm inheight, constructed as shown in FIGS. 1 and 2.

This nonaqueous liquid electrolyte secondary battery was subjected to aninitial charge process by the following procedure. First, the batterywas left standing at a high temperature environment at 45° C. for 2hours and then, under the same environment, charged up to 4.2 V at 0.2 C(120 mA) with a constant current and voltage for 15 hours. Thereafter,it was discharged down to 3.0 V at 0.2 C and again charged for thesecond cycle under the same conditions as the first cycle to produce anonaqueous liquid electrolyte secondary battery.

In order to investigate the charge/discharge cycle characteristics ofthe nonaqueous liquid electrolyte secondary battery obtained, cycleseach consisting of charging up to 4.2 V at 1 C rate with a constantcurrent and voltage at 45° C. for 3 hours and discharging down to 2.7 Vat 1 C rate were repeated and the capacity retention ratio after 300cycles was measured. The characteristics of the battery for Example 1are shown in Table 1, wherein viscosity (cP) refers to a value at 20° C.The battery thickness at the first cycle is referred to as a referenceand the increase in thickness of the outer packaging at 800th cycle ispresented in Table 1

Examples 2 to 9

The same procedure as that of Example 1 was followed, except that theamount, molecular weight and viscosity of the macromolecules to be addedto the liquid electrolyte were altered as shown in Table 1, to prepare athin, nonaqueous liquid electrolyte secondary battery for makingevaluations. The characteristics of the battery for each Example areshown in Table 1.

Comparative Example 1

The same procedure as that of Example 1 was followed, except that amixed solvent of BL and EC (volume ratio 75:25) to which 1.5 mol/l ofLiBF₄ was dissolved was used for the nonaqueous liquid electrolyte, toprepare a thin, nonaqueous liquid electrolyte secondary battery formaking evaluations. The characteristics of the battery for ComparativeExample 1 are shown in Table 1. TABLE 1 Increase in thickness AmountCapacity of outer Molecular (% by Viscosity retention packaging weightweight) (cP) ratio (%) (mm) Example 1 5 × 10⁶ 0.7 1100 85 0.25 Example 25 × 10⁶ 5 20000 90 0.21 Example 3 1 × 10⁷ 0.01 5000 88 0.23 Example 4 1× 10³ 9 60 80 0.7 Example 5 3 × 10³ 0.05 7 50 0.8 Example 6 8 × 10⁵ 6500 84 0.35 Example 7 9 × 10⁷ 0.9 30000 93 0.15 Example 8 1 × 10⁶ 0.1700 85 0.32 Example 9 5 × 10⁴ 2 200 82 0.38 Comparative — — 6.6 15 0.95Example 1

As apparent from Table 1, the batteries according to Examples 1 to 9each having a nonaqueous liquid electrolyte containing a nonaqueoussolvent to which macromolecules were added to increase the viscosity ofthe liquid electrolyte can remarkably improve the capacity retentionratio after 300 cycles at 45° C. In addition, in Examples 1 to 9, a loadof 300 kg per cell was applied, to show that no leakage of thenonaqueous liquid electrolyte from the obtained nonaqueous liquidelectrolyte secondary battery was observed.

Furthermore, it is shown that swelling of the outer packaging issuppressed, meaning there is very little deformation of the electrodeassembly. The deformation of electrodes is supposed to be one of thecauses influencing the capacity retention ratio and, in these Examples,the capacity retention ratio is higher when the viscosity is higher.

Example 10

This Example used the same battery construction as Example 1 except thepreparation of nonaqueous liquid electrolyte.

(Preparation of Nonaqueous Liquid Electrolyte)

Into a mixed solution of ethylene carbonate (EC) and γ-butyrolactone(BL) (volume ratio 25:75) 1.5 mol/l of lithium borofluoride (LiBF₄) wasdissolved to prepare a nonaqueous liquid electrolyte. Then, 1% byweight, on the basis of the nonaqueous liquid electrolyte, ofpolyethylene oxide having a molecular weight of 4,000,000 was added withstirring to the liquid electrolyte to prepare a nonaqueous liquidelectrolyte.

This nonaqueous liquid electrolyte showed a decrease in its viscosityaccording to the increase of shear rate at 20° C. Specifically, theviscosity was 1,000 cP when the shear rate was 20 S and decreased downto 300 cP when the shear rate was 150 S.

In order to investigate the change ratio in battery thickness before andafter charge/discharge cycles in the nonaqueous liquid electrolytesecondary battery obtained, cycles each consisting of charging up to 4.2V at 2 C rate with a constant current and voltage at 45° C. for 3 hoursand discharging down to 3.0 V at 1 C rate were repeated and thethickness of the battery after 500 cycles was measured as d₅₀₀ and thechange ratio in battery thickness was measured as (d₅₀₀-d₀)/d. Thecapacity retention ratio was also measured. The change ratio in batterythickness for Example 10 is shown in Table 2.

Examples 11 to 16

The same procedure as that of Example 8 was followed, except that themolecular weight and viscosity of the macromolecules to be added to theliquid electrolyte were altered as shown in Table 2, the amountremaining the same, to prepare a thin, nonaqueous liquid electrolytesecondary battery for making evaluations. The characteristics of thebattery for each Example are shown in Table 2.

Comparative Example 2

The same procedure as that of Example 10 was followed, except that amixed solvent of BL and EC (volume ratio 75:25) to which 1.5 mol/l ofLiBF₄ was dissolved was used for the nonaqueous liquid electrolyte, toprepare a thin, nonaqueous liquid electrolyte secondary battery formaking evaluations. The characteristics of the battery for ComparativeExample 2 are shown in Table 2. TABLE 2 Apparent Shear Average viscosityrate molecular (d₅₀₀ − d₀)/ (cP) (S⁻¹) weight d₀ Example 10 2000 150   4× 10⁶ 4.1 Example 11 150 250 9.7 × 10⁸ 2.3 Example 12 7 1000 8.6 × 10³6.1 Example 13 50 500 4.5 × 10⁴ 5.4 Example 14 1000 100 6.2 × 10⁷ 3.3Example 15 8000 70 7.1 × 10⁵ 3.4 Example 16 10000 20 3.4 × 10⁶ 2.1Comparative 6.6 350 — 36 Example 2

As apparent from Table 2, the secondary batteries according to Examples10 to 16 each having a nonaqueous liquid electrolyte containing anonaqueous solvent to which macromolecules were added to increase theviscosity of the liquid electrolyte can remarkably reduce the change inthickness of the batteries after 500 cycles at 45° C.

Example 17

This Example used the same battery construction as that of Example 1except the preparation of nonaqueous liquid electrolyte.

(Preparation of Nonaqueous Liquid Electrolyte)

Into a mixed solvent of ethylene carbonate (EC) and y-butyrolactone (BL)(volume ratio 25:75) 2.5 mol/l of lithium borofluoride (LiBF₄) wasdissolved to prepare a nonaqueous liquid electrolyte. Then, 0.5% byweight, on the basis of the nonaqueous liquid electrolyte, ofpolyethylene oxide having a molecular weight of 7,000,000 was added withstirring to the liquid electrolyte to prepare a nonaqueous electrolyte.

(Preparation of Electrode Assembly)

A strip-like positive electrode lead was welded to the collector of thepositive electrode and a strip-like negative electrode lead was weldedto the collector of the negative electrode. Then, the positive andnegative electrodes were spirally wound with the separator between them,followed by shaping into a flattened form to fabricate an electrodeassembly. A laminated film, 90 μm in thickness, made of an aluminum foilcovered on both sides with polypropylene was shaped into a sack, intowhich the electrode assembly was accommodated in such a manner that thelaminated surface shown in FIG. 2 could be seen through the opening ofthe sack. The nonaqueous liquid electrolyte was injected into theelectrode assembly in the laminated film so that the amount reached 4.5g per Ah of battery capacity to assemble a thin, nonaqueous liquidelectrolyte secondary battery, 3 mm in thickness, 40 mm in width and 70mm in height, constructed as shown in FIGS. 1 and 2.

The nonaqueous liquid electrolyte secondary battery was subjected to aninitial charge process by the following procedure. First, the batterywas left standing at a high temperature environment at 45° C. for 2hours and then, under the same environment, charged up to 4.0 V at 0.2 C(120 mA) and then up to 4.2 V at 0.05 C (30 mA) with a constant currentand voltage for 15 hours. Thereafter, it was discharged down to 3.0 V at0.2 C and again charged for the second cycle under the same conditionsas those for the first cycle to produce a nonaqueous liquid electrolytesecondary battery.

In order to investigate the change ratio of thickness of the batterybefore and after the charge/discharge cycles in the nonaqueouselectrolyte secondary battery obtained, cycles each consisting ofcharging up to 4.2 V at 2 C rate with a constant current and voltage at25° C. for 3 hours and discharging down to 3.0 V at 1 C rate wererepeated.

The resistivity change ratio (R₅₀₀—Ro)/Ro, wherein Ro represents abattery resistivity before discharging and R₅₀₀ represents a batteryresistivity after 500 cycles, was measured. The resistivity change ratioof the battery for Example 17 is shown in Table 3.

Examples 18 to 23

The same procedure as that of Example 17 was followed, except that themolecular weight and the ratio of ion conductivity σ (10⁻³ S/cm) toviscosity η (cP), p, of the macromolecules to be added to the liquidelectrolyte were altered as shown in Table 3, the amount remaining thesame, to prepare a thin, nonaqueous liquid electrolyte secondary batteryfor making evaluations. The characteristics of the battery for eachExample are shown in Table 3.

Comparative Example 3

The same procedure as that of Example 17 was followed, except that amixed solvent of BL and EC (volume ratio 75:25) to which 1.5 mol/l ofLiBF₄ was dissolved was used for the nonaqueous liquid electrolyte, toprepare a thin, nonaqueous liquid electrolyte secondary battery formaking evaluations. The characteristics of the battery for ComparativeExample 3 are shown in Table 3. TABLE 3 P Average molecular weight (R₅₀₀− R₀)/R₀ Example 17 0.2   7 × 10⁶ 2.1 Example 18 0.03 2.3 × 10⁵ 2.3Example 19 0.002 5.5 × 10⁷ 1.4 Example 20 0.08 4.7 × 10³ 4.4 Example 210.009 3.8 × 10⁶ 1.9 Example 22 0.05 6.9 × 10⁴ 3.1 Example 23 0.0003 7.4× 10⁸ 1.3 Comparative 3.3 — 10 Example 3

As apparent from Table 3, the secondary batteries according to Examples17 to 23 each having a nonaqueous liquid electrolyte containing anonaqueous electrolyte wherein the ratio of ion conductivity σ (10⁻³S/cm) to viscosity η (cP), p (σ/η), is <0.1 in the nonaqueous liquidelectrolyte can remarkably reduce the change in battery resistivityafter 500 cycles at 25° C. As described above, it is possible accordingto the present invention to provide a nonaqueous liquid electrolytesecondary battery which suppresses leakage and ensures high dischargecharacteristics. In addition, it is possible to provide a nonaqueousliquid electrolyte secondary battery which reduces the unevenness ofliquid electrolyte and lessens the change of electrodes. Furthermore, itis possible to provide a nonaqueous liquid electrolyte secondary batterywhich reduces the unevenness of liquid electrolyte and lessens thechange in battery resistivity.

1-15. (canceled)
 16. A non-polymerized nonaqueous liquid electrolytehaving a viscosity at 20° C. of 60 cP to 30,000 cP comprising: anonaqueous solvent, an electrolyte dissolved in the nonaqueous solventcontaining y-butyrolactone, and a macromolecular material added to thenonaqueous solvent comprising the structure represented by the formula:

CH₂—CH₂—O

_(n) wherein n≧1, wherein the content of the macromolecular materialadded to the nonaqueous solvent if 0.01% or more, but less than 10% byweight.
 17. The non-polymerized nonaqueous liquid electrolyte, which isformulated for use in a liquid electrolyte secondary battery having apositive electrode containing an active material, a negative electrodecontaining a material which absorbs and desorbs lithium ions, and aliquid electrolyte sandwiched between the positive and negativeelectrodes.
 18. A secondary battery comprising the non-polymerizednonaqueous liquid electrolyte of claim
 16. 19. The secondary battery ofclaim 18, wherein the ratio p ranges from 0.001 to 0.05, wherein =σ/η,and σ is the ion conductivity (10⁻³ S/cm) and η is the viscosity (cP).20. The secondary battery of claim 18, which has a capacity retentionratio ranging from 50-90% after 300 cycles of charging up to 4.2 V at arate of 1 C with a constant current and voltage at 45° C. for 3 hoursand discharging down to 2.7 V at a rate of 1 C.
 21. The secondarybattery of claim 18, which exhibits a (d₅₀₀−d₀)/d₀ ratio of less than 36after 500 cycles of charging up to 4.2 V at a rate of 2 C with aconstant current and voltage at 45° C. for 3 hours and discharging downto 3.0 V at a rate of 1 C, wherein d₅₀₀ is the thickness of the batteryafter 500 cycles of charging and discharging and d₀ is the originalbattery thickness.
 22. The secondary battery of claim 18, which shows noleakage of the nonaqueous liquid electrolyte after application of a loadof 300 kg per cell.
 23. A method for making a secondary batterycomprising: fabricating a secondary battery comprising a negativeelectrode, positive electrode, an electrolyte, and an outer packagingmaterial, wherein the electrolyte is the non-polymerized nonaqueousliquid electrolyte of claim 16.